This invention relates to multi-channel x-ray data acquisition equipment for use in computerized transverse axial tomography systems. More specifically, this invention relates to electronic circuits for acquiring and digitizing x-ray transmission data along a plurality of ray paths and to circuits and methods for calibrating such equipment and for compensating the data for drift in channel gain and dc offset parameters.
Shadow-free images of internal body structures may be produced by the methods of computerized transverse axial tomography. In a typical application of those methods, a moving x-ray source is repeatedly pulsed to transmit x-ray energy along a plurality of distinct ray paths through a body undergoing examination. Energy transmitted through the body is detected in an array of x-ray sensors and interpreted, typically by use of a digital computer, to produce x-ray images of internal body sections. The methods of computerized tomography are more specifically described, for example, in U.S. Pat. No. 3,778,614 to Hounsfield which is incorporated in this disclosure, by reference, as background material.
Modern tomography equipment typically comprises arrays of hundreds of X-ray sensors (for example, ionization chambers) which function to simultaneously measure x-ray transmission over a large number of ray paths. Information from each sensor typically must be amplified, conditioned, and digitized to permit the calculation and generation of image data in a digital computer. During a typical tomographic examination, the x-ray source and sensor array rotate and/or translate about the body to acquire data from hundreds of x-ray projections in a few seconds. X-ray data from each sensor must be transmitted to the computer with an accuracy on the order of one part in 2.sup.14 or more to permit the production of high resolution images. The electronic gain and voltage offset in each of the hundreds of data channels are, however, subject to drift; for example, under conditions of temperature change or mechanical motion. It is, therefore, necessary to continuously and automatically calibrate each data acquisition channel during a tomographic examination procedure.